The approaches described in this section are approaches that could be pursued, but not necessarily approaches that have been previously conceived or pursued. Therefore, unless otherwise indicated, it should not be assumed that any of the approaches described in this section qualify as prior art merely by virtue of their inclusion in this section.
Operators of printing presses and other print devices or systems (hereinafter collectively referred to as “print systems”) may rely upon various print tests to calibrate or diagnose problems with their print systems. For example, operators may perform such tests on a periodic basis, or before major print jobs, so as to ensure that print jobs do not suffer from problems such as ink imbalances or mechanical defects. One such class of tests involves printing one or more “diagnostic targets” having color or grayscale patches. The operator may utilize the target to visually recognize color problems based on personal experience and/or comparisons to other targets printed under optimal circumstances. The operator may then take appropriate corrective action.
In some embodiments, the analysis of a printed diagnostic target may be automated. Colorant usage on the printed target may be measured via, for example, a digital scanning system, to arrive at measured color values. The measured values may then be compared to baseline values indicating expected measurements for the target when printed under optimal circumstances. Based on these comparisons, “deltaE” values may be computed for each printed color. An analysis of the deltaE values can identify color imbalances as well as appropriate corrective measures.
An example of such an analysis of a diagnostic target is described in U.S. Pat. No. 7,760,397, issued Jun. 2, 1998 and entitled “Calibration sheet and method of calibrating a digital printer,” the contents of which are hereby incorporated by reference for all purposes as if set forth in their entirety. Quality control images are printed and then scanned for analysis. When differences between measurements and expectations are identified, the differences are evaluated to create quality control metrics. Before any digital image is printed, a print system may filter the digital image using the quality control metrics to compensate for the color discrepancies.
Print systems print diagnostic targets based on a digital data representative of the diagnostic target, such as a PostScript or PDF document. This digital data is hereinafter referred to as a template for a diagnostic target, or the target template. The template specifies color values that a print system should print in any of a variety of color metric systems hereinafter referred to a color spaces. The print system utilizes the specified color values to determine how much colorant to apply to any given point when printing the diagnostic target. As used herein, colorant refers to any discrete color of ink that a print system is capable of applying to a page. A color of ink is discrete when the printer can print the color without mixing ink sources. For example, many printers are capable of applying four colorants—cyan, magenta, yellow, and black.
One technique for generating useful target templates involves generating a large number of candidate target templates and then selecting one or more of the candidate target templates that meet certain criteria. For each candidate target template, a grid of colored patches is generated. The grid is created by first selecting a set of colored patches that are to be tested. The selection of patch color values for color quality control purposes may occur in a variety of manners, including using well known sets of colors and/or optimizing colors for specific print systems or print jobs. The grid is then populated by randomly or pseudo-randomly selecting the position within the grid for each patch in the set of color patches.
While the above techniques greatly simplify the process of identifying color defects, other defects that may result in print performance issues are more difficult to identify through conventional techniques. One such class of defects, collectively referred to herein as spatial defects, causes print performance issues that are localized to particular regions of a printed page. For example, a defective print system may print a region in the bottom left corner of a page more lightly than the rest of the page. Or, the defective print system may print vertical streaks of color across the page. Many spatial issues do not manifest themselves obviously on each and every printed page, but rather appear under only certain circumstances—such as in pages having certain balances of colors or in pages with photographs heavy in contrast. To be certain that such issues do not exist, one would typically need to print large quantities of different test jobs, at a potentially prohibitive expense in terms of time or money. Thus, operators may elect not to test for spatial issues, and thus spatial defects may not be detected until a defective print job has already been performed.